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Eigenfrequency with rigid motion suppression node
Posted 01.04.2022, 13:02 GMT-4 Piezoelectric Devices, Structural & Acoustics, Acoustics & Vibrations Version 5.3a 2 Replies
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Hi,
I'm doing the study of piezoelectric material and facing a problem now. At the very beginning, I understand that if a material doesn't have any constrain, then the first 6 eigenfrequencies will representate 6 rigid body motion and will be very small or be imaginary numbers, but now I calculate the first 12 eigenfrequencies with two model, one is applied rigid motion suppression node and one is not, I thought the model without rigid motion suppression node will have 6 elastic eigenfrequencies after 6 rigid eigenfrequencies, and should be equal to the first 6 eigenfrequencies that obtained by the model which applied rigid motion suppression node, but the result is not, none of the eigenfrequencies is equal.
Is my logic wrong or there is something that need to be considered about?
And I also have second one problem, if I have to calculate the eigenfrequencies around 1 MHz, should the model applied rigid motion suppression or not? Or, wether the rigid motion suppression node influences the eigenfrequencies after the very first 6 eigenfrequencies?
Thank you in advanced.
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Hello 宇宣,
What the Rigid Motion Suppression node does behind the scenes is that it imposes point constraints that just eliminate the 6 rigid body motions that a 3D structure would otherwise have (See this blog post for more information). The eigenmodes of the thus-constrained structure will respect those constraints and therefore depend on those constraints.
You can think about it this way also:
There's more than one way you can pick point constraints to eliminate the 6 rigid body motions. While the Rigid Motion Suppression feature in COMSOL picks a particular set of point constraints, you could certainly manually pick a different set of points to impose constraints on to eliminate those 6 rigid body motions (or use the same points but impose different constraints). Each such set of constraints will result in a different set of eigenmodes.
To obtain the eigenmodes of your structure, you should apply constraints that correspond to the real-life situation, not use Rigid Motion Suppression.
Best,
Jeff
-------------------Jeff Hiller
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Hi Jeff,
Thank you so much, you solved my big problem!
But I have one more question here.
I have a piezo disc and I want to get its eigenfrequencies, since the disc is operating in the water tank, should I model the disc with water domain, and run the eigenfrequency study for piezoelectricity and acoustic-structural coupling multiphysics or just model the disc? And if the water domain is necessary, should I add the PML since the water tank compare to the disk is infinite domain?
Thank you again!
Hello 宇宣,
What the Rigid Motion Suppression node does behind the scenes is that it imposes point constraints that just eliminate the 6 rigid body motions that a 3D structure would otherwise have (See this blog post for more information). The eigenmodes of the thus-constrained structure will respect those constraints and therefore depend on those constraints.
You can think about it this way also:
There's more than one way you can pick point constraints to eliminate the 6 rigid body motions. While the Rigid Motion Suppression feature in COMSOL picks a particular set of point constraints, you could certainly manually pick a different set of points to impose constraints on to eliminate those 6 rigid body motions (or use the same points but impose different constraints). Each such set of constraints will result in a different set of eigenmodes.
To obtain the eigenmodes of your structure, you should apply constraints that correspond to the real-life situation, not use Rigid Motion Suppression.
Best,
Jeff
Jeff Hiller